A new study of British civil servants shows that cognitive skills such as memory and reasoning are already declining, typically, among people as young as 45.

Researchers from France and England looked at data from the Whitehall II cohort — a group of roughly 10,000 government employees in London who have been studied since the mid-1980s. Through the years, study participants have answered surveys and given information about all aspects of their lives, from diet to education, as well as agreed to undergo special medical exams.

They’ve also completed several cognition tests involving quizzes and problems to solve on three different occasions each spaced five years apart: in 1997-99, 2002-04, and 2007-09. For those tests, the Whitehall II participants answered questions that measured their verbal and mathematical reasoning. They were quizzed on their memory and asked, for example, to remember as many words as possible from a long list. They were also asked to name, respectively, as many words as possible that begin with the letter “S,” for instance, or to name as many animals as possible.

The results of all that testing are published this week in the journal BMJ (formerly called the British Medical Journal). They show that, on average, performance on the cognition tasks worsened as the subjects got older. The declines were steepest among the oldest participants, who were aged 65-70 at the beginning of the 10-year study period in 1997-99. But even for the youngest participants, aged 45-49 in 1997-99, average skills declined with age in every one of the test categories except vocabulary.

The results are interesting, the study authors suggest, because they show that cognitive decline may start much earlier than previously realized. They note that past research has failed to find much evidence for cognitive decline before age 60. However, this may be because the tests used to measure mental prowess among older people are too easy for people in middle age — “that is, simple tests [lead] to little variability in scores” for younger people, the researchers write.

The authors add:

The age at which cognitive decline begins is important because behavioural or pharmacological interventions designed to alter cognitive ageing trajectories are more likely to work if they are applied when individuals first begin to experience decline.

In other words, if we can find out when exactly cognition first starts to falter, perhaps we’ll be able to do more to keep ourselves sharp and to stop our skills from declining further.